Cha zeolite membrane and method of preparing the same

ABSTRACT

Disclosed are a CHA zeolite membrane and a method of preparing the same, and more particularly, a CHA zeolite membrane having high capacity to separate CO 2 /N 2  and CO 2 /CH 4  even under wet conditions using a membrane produced using a synthetic precursor having a controlled ratio of Si and Al, a method of preparing the same, and a method of capturing and removing carbon dioxide using the membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional of U.S. patent application Ser. No.16/729,587 filed on Dec. 30, 2019, which claims the benefit under 35 USC119(a) of Korean Patent Application No. 10-2018-0172311, filed on Dec.28, 2018, in the Korean Intellectual Property Office, the entiredisclosures of which are incorporated herein by reference for allpurposes.

TECHNICAL FIELD

The present invention relates to a CHA zeolite membrane and a method ofpreparing the same, and more particularly to a CHA zeolite membranehaving high capacity to separate CO₂/N₂ and CO₂/CH₄ even under wetconditions using a membrane produced using a synthetic precursor havinga controlled ratio of Si and Al, a method of preparing the same, and amethod of capturing and removing carbon dioxide using the membrane.

BACKGROUND ART

Zeolites are aluminosilicate crystalline materials that have regular andrigid microporous structures. Because the size of the pore structures isclose to those of permanent gases, a zeolite, if appropriately selected,can act as a molecular sieve, functioning analogously to a conventionalsand/stone/gravel sieve used on construction sites. Indeed, zeolitemembranes have high potential for use in distinguishing minutedifferences between the shape and size of permanent gases, thusachieving energy-efficient gas separation (Jeon, M. Y. et al., Nature2017, 543, 690-691; Tung, C. T. P. et al., Science 2011, 334,1533-1538).

Separation of mixed material using a zeolite membrane is based on thedifference in permeation rates of the constituents of the mixed materialpermeating the membrane. In the zeolite membrane, permeation is achievedby adsorption, diffusion and desorption. The intrinsic pore structureand surface properties of the zeolite have an influence on this process.In this respect, CHA zeolites with a pore size of 0.37×0.42 nm² have astructure suitable for separating CO₂ having a size of 0.33 nm from N₂(0.364 nm) and CH₄ (0.38 nm).

If a zeolite is chosen such that its pore size lies between the size ofCO₂ and other larger molecules, the molecular sieve can act as a goodCO₂ separator. In this respect, eight-membered-ring (8 MR) zeolites,whose maximum pore size is 0.43 nm, are promising molecular sieves forthe separation of CO₂ (0.33 nm) from other larger gas molecules such asN₂ (0.364 nm) and CH₄ (0.38 nm) (Breck, D. W., John Wiley & Sons, Inc.:1974). Effective CO₂/N₂ and CO₂/CH₄ separation is attractive to manyresearchers because of the importance of regulating flue gas emissionsin fossil fuel combustion and in the upgrading of natural gas andbiogas. Research is underway to realize high CO₂/N₂ and CO₂/CH₄separation performance of CHA zeolite membranes (Avila et al., Journalof Membrane Science, 2009, 335, 32-36; Li et al., Industrial andEngineering Chemistry Research, 2010, 49, 4399-4404).

Conventional zeolite membranes are generally measured under conditionsthat do not contain moisture in the feed gas. The reason for this isthat the moisture contained in the feed gas affects the performance ofthe membrane and thus prevents smooth gas separation through themembrane. In actual gas separation processes, however, the membrane isoften exposed to wet conditions. Examples thereof include flue gastreatment and biogas upgrading. Flue gas generated after combustion offossil fuel undergoes a desulfurization process. In this process, alarge amount of water is injected into flue gas to finally have acomposition of CO₂, N₂ and H₂O. Since biogas is produced from livingorganisms, it inevitably contains a large amount of water and has acomposition of CO₂, CH₄ and H₂O. Therefore, in order to use conventionalzeolite membranes for the actual flue gas treatment and biogas upgradingprocesses, it is necessary to completely remove the water contained inthe feed gas through a dehydration process. This dehydration process isa major factor that increases the cost of the process and thus limitsthe application of the membrane process to the actual process.

In flue gas treatment or biogas upgrading processes that requireselective separation of CO₂, moisture is always present, in addition toCO₂ and N₂, or CO₂ and CH₄. Moisture present in the mixed gas serves tohinder the selective adsorption and diffusion of CO₂ in the zeolitemembrane, resulting in a reduction in CO₂ permeance. In order to preventthe decreased CO₂ permeance due to moisture, a method of synthesizingzeolite such that the surface of zeolite is hydrophobic has beenproposed (Kosinov et al., Journal of Materials Chemistry A, 2014, 2,13083-13092). Zeolite has a structure in which Si and Al are connectedthrough four crosslinked oxygen atoms. In this regard, when the ratio ofAl in the structure is lowered, the surface becomes hydrophobic. Thehydrophobic surface has been reported to be less susceptible to moistureand thus result in higher permeance.

When the zeolite membrane is fabricated, defects having a size largerthan zeolite pores are also formed. These defects provide non-selectivepassages, thus reducing the selectivity of the membranes. However, it isknown that in the presence of moisture, moisture may be adsorbed on thedefects and thus contribute to the improvement of separation performance(Hong et al., Chemistry of Materials, 2018, 30, 3346-3358). For thisreason, in order to obtain a membrane having optimum separationperformance under wet conditions, the defect structures of membranes aswell as the surface properties associated with water adsorption shouldbe considered.

Since Si and Al serve as central atoms for forming zeolite structures,the formation of the zeolite structures is greatly affected by the ratioof the two atoms. Thus, the CHA zeolite membranes exhibit variousresults depending on the Si/Al ratio, thus causing the formation ofvarious defect structures.

PCT Patent Publication No. WO 2016/006564 A1 discloses a method ofpreparing a membrane while changing Si/Al. However, the document relatesto a membrane for gas separation under dry conditions, wherein themembrane has a problem in which the membrane is not optimized under thecondition of a feed gas containing moisture, which is an actual processcondition.

Thus, there is a need for a technology for producing a membrane that canexhibit high performance under such actual process conditions containingmoisture and can improve economic efficiency.

Therefore, as a result of extensive efforts to solve these problems, thepresent inventors found that, in the preparation of a CHA zeolitemembrane, when preparing a membrane using a synthetic precursor having acontrolled ratio of Si and Al, unlike conventional zeolite membranes,the hydrophobicity and defect formation of the zeolite membranes areoptimized, and thus excellent CO₂/N₂ and CO₂/CH₄ separation performancecan be obtained even under wet conditions. Based on this finding, thepresent invention has been completed.

DISCLOSURE OF INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide amembrane having higher CO₂/N₂ and CO₂/CH₄ separation performance thanconventional zeolite membranes in both dry and wet conditions and amethod of preparing the same.

It is another object of the present invention to provide a method ofseparating CO₂ using the membrane.

In accordance with an aspect of the present invention, the above andother objects can be accomplished by the provision of a method ofpreparing a CHA zeolite membrane including adding a support including aCHA particle seed layer deposited thereon to a synthetic solution forgrowing zeolite containing an organic structure-directing agent and Siand Al sources, and performing hydrothermal synthesis, wherein a molarratio of Si/Al in the synthetic solution for growing zeolite is 5 ormore.

In accordance with another aspect of the present invention, there isprovided a method of preparing a CHA zeolite membrane including adding asupport including a CHA particle seed layer deposited thereon to asynthetic solution for growing zeolite containing an organicstructure-directing agent, and a Si source, without containing an Alsource, and performing hydrothermal synthesis.

In accordance with another aspect of the present invention, there isprovided a CHA zeolite membrane produced by the method, wherein the CHAzeolite membrane has a porosity of a defect of 0.1 to 1% and a z-axistortuosity (tortuosity in a thickness direction) of the defect of 1 to2.

In accordance with another aspect of the present invention, there isprovided a method of separating CO₂ from a mixture containing CO₂ and amolecule selected from the group consisting of CH₄, N₂, O₂, C₂H₄, C₂H₆,C₃H₆ and C₃H₈ using the CHA zeolite membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains a least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a SEM image showing a membrane according to one embodiment ofthe present invention;

FIG. 2 shows graphs of the CO₂/N₂ permeance of the membrane according toone embodiment of the present invention;

FIG. 3 shows graphs of the CO₂ and N₂ permeance of the membraneaccording to one embodiment of the present invention;

FIG. 4 shows graphs of the permeance and CO₂/CH₄ SFs for a mixture withCO₂ and CH₄ equimolar ratios through (a)-(b) CHA_100 and (c)-(d) CHA_∞according to one embodiment of the present invention;

FIG. 5 shows graphs of (a) CO₂ permeance×CO₂/N₂ SF and (b) CO₂permeance×CO₂/CH₄ SF expressed as a function of temperature according toone embodiment of the present invention;

FIG. 6 shows FCOM images of a cross-section and a top surface of themembrane according to one embodiment of the present invention;

FIG. 7 shows cross-sectional SEM (1 column) and FCOM (2-4 columns)images of (a1)-(a4) CHA_20 (top), (b1)-(b4) CHA_100 (middle) and(c1)-(c4) CHA_∞ (bottom) measured at the same vertical magnification ofthe membrane according to one embodiment of the present invention;

FIG. 8 shows top SEM (1 column) and FCOM (2-4 columns) images of(a1)-(a4) CHA_20 (top), (b1)-(b4) CHA_100 (middle) and (c1)-(c4) CHA_∞(bottom) measured at the same vertical magnification of the membraneaccording to one embodiment of the present invention;

FIG. 9 shows the porosity of the non-zeolite portion (that is, defect)and the zeolite portion with corresponding effects on the total CO₂molar flux regarding the CHA_20, _100,_∞, and post-treated membrane(defect-free state) according to an embodiment of the present invention;

FIG. 10 is a graph showing estimated CO₂ permeance based on the resultsof defect blocking (vertical pattern) and surface inhibition (diagonalpattern) according to one embodiment of the present invention;

FIG. 11 is a graph showing (a) CO₂/N₂ SF vs. CO₂ permeance and (b)CO₂/CH₄ SF vs. CO₂ permeance of CHA_100 and _∞ membrane compared tothose of other zeolite membranes according to an embodiment of thepresent invention;

FIG. 12 shows scanning electron microscope (SEM) results of (a) Si-CHAparticles and (b) Si-CHA seed layers of the membrane and (c) XRD resultscorresponding thereto according to one embodiment of the presentinvention;

FIG. 13 shows CO₂/N₂ permeance and CO₂/N₂ SF of (a) CHA_100 and (b)CHA_∞ depending on the temperature of a membrane when a wet gas mixturehaving a composition of 14.7 kPa CO₂/83.3 kPa N₂/3 kPa H₂O is flowedaccording to one embodiment of the present invention;

FIG. 14 is an image obtained after dropping water on (a) CHA_20 (left),(b) CHA_100 (middle), and (c) CHA_∞ (right) membrane according to oneembodiment of the present invention;

FIG. 15 is a cross-sectional FCOM image of (a1) CHA_20, (b1) CHA_100,and (c1) CHA_∞ membrane according to one embodiment of the presentinvention;

FIG. 16 is cross-sectional SEM and FCOM images of (a1)-(a3) CHA_100 and(b1)-(b3) CHA_∞ membrane according to one embodiment of the presentinvention;

FIG. 17 is an image obtained after dropping dye on (a) CHA_20 (left),(b) CHA_100 (middle), and (c) CHA_∞ (right) membrane according to oneembodiment of the present invention; and

FIG. 18 is a tilted image (left) and a top image (right) obtained byperforming image processing on FCOM images of (a1)-(a2) CHA_20,(b1)-(b2) CHA_100 and (c1)-(c2) CHA_∞ membrane shown in FIG. 6 accordingto one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as appreciated by those skilled in the field towhich the present invention pertains. In general, the nomenclature usedherein is well-known in the art and is ordinarily used.

In the production of a CHA zeolite membrane, it was found that, when amembrane is fabricated using a synthetic precursor having a controlledratio of Si and Al, the zeolite membrane has optimized hydrophobicityand defect formation and thus high separation performance of CO₂/N₂ andCO₂/CH₄ even under wet conditions, unlike conventional zeolitemembranes.

Therefore, in one aspect, the present invention provides a method ofpreparing a CHA zeolite membrane including adding a support including aCHA particle seed layer deposited thereon to a synthetic solution forgrowing zeolite containing an organic structure-directing agent, and Siand Al sources, and performing hydrothermal synthesis, wherein a molarratio of Si/Al in the synthetic solution for growing zeolite is 5 ormore.

Therefore, in another aspect, the present invention provides a method ofpreparing a CHA zeolite membrane including adding a support including aCHA particle seed layer deposited thereon to a synthetic solution forgrowing zeolite containing an organic structure-directing agent, and aSi source, without containing an Al source, and performing hydrothermalsynthesis.

In the present invention, the molar ratio of Si/Al may be 5 or more,preferably 5 to 1,000, more preferably 10 to 1000, 20 to 1,000, 10 to500, and most preferably 20 to 200.

In the present invention, the properties of zeolite are changeddepending on the Si/Al ratio of the synthetic solution. Thus, similarly,zeolites having a CHA structure are affected by the Si/Al ratio of thesynthetic solution. CHA particles with a low Si/Al ratio are hydrophilicand have many defects, whereas CHA particles with a high Si/Al ratio arehydrophobic and have fewer defects. In the present invention, thehydrophobicity and defect formation of the membrane can be optimized bycontrolling the Si/Al ratio when synthesizing the CHA zeolite membranebased on this principle. This results in the production of a CHA zeolitemembrane having optimal Si/Al content providing excellent CO₂ separationperformance under wet conditions.

Since the Si/Al ratio of the membrane affects not only thehydrophobicity of the membrane but also the formation of the defectstructure, it is important to find a Si/Al ratio optimal for CO₂separation in a wet condition. In the present invention, CHA zeolitemembranes with various Si/Al ratios are produced, and the CO₂/N₂ andCO₂/CH₄ separation performance of each membrane under wet conditions areidentified. As a result, the conditions for the synthesis of CHA zeolitemembranes that exhibit the best performance under wet conditions werefound.

As the temperature increases, adsorption of water on the membranedecreases. For this reason, the high-temperature wet condition isrelatively less affected by water than the low-temperature wetcondition. Therefore, a CHA zeolite membrane having optimumtemperature-dependent performance is present. This is determined by theSi/Al ratio, which is directly related to the hydrophobicity and defectformation of the membrane. For this reason, it is important to find theoptimum Si/Al ratio depending on the temperature. In the presentinvention, a membrane having an optimized Si/Al ratio depending on theprocess temperature was produced, so that a CHA zeolite membranespecialized for the temperature of each process may be applied.

In the present invention, the support may include at least one selectedfrom the group consisting of alumina, polypropylene, polyethylene,polytetrafluoroethylene, polysulfone, polyimide, silica, glassgamma-alumina, mullite, zirconia, titania, yttria, ceria, vanadia,silicon, stainless steel and carbon.

In the present invention, the organic structure-directing agent mayinclude one or more selected from the group consisting of TMAdaOH(N,N,N-trimethyl adamantylammonium hydroxide), TMAdaBr (N,N,N-trimethyladamantylammonium bromide), TMAdaF (N,N,N-trimethyl adamantylammoniumfluoride), TMAdaCl (N,N,N-trimethyl adamantylammonium chloride), TMAdaI(N,N,N-trimethyl adamantylammonium iodide), TEAOH (tetraethylammoniumhydroxide), TEABr (tetraethylammonium bromide), TEAF (tetraethylammoniumfluoride), TEACl (tetraethylammonium chloride), TEAI (tetraethylammoniumiodide), dipropylamine and cyclohexylamine.

In the present invention, the synthetic solution for growing zeolitegrowth may have a molar ratio of TMAdaOH: SiO₂: H₂O: NaOH: Al(OH)₃ of 1to 100: 100: 1000 to 20000: 1 to 100: 0 to 20.

In the present invention, hydrothermal synthesis may be carried out at atemperature of 100 to 200 ° C. for 12 to 240 hours.

Typical CHA zeolites are promising for separating CO₂ from largemolecules such as N₂ (associated with carbon capture after combustion)and CH₄ (associated with natural gas/biogas upgrading). In particular,the pore size of CHA zeolites (0.37×0.42 nm²) can distinguish sizedifferences between small molecules such as CO₂ (0.33 nm) and larger N₂(0.364 nm) or CH₄ (0.38 nm) molecules, and thus can separate CO₂ throughCHA membranes. In addition, the siliceous component of the CHA zeolitecan reduce the adsorption capacity of smaller H₂O molecules (0.265 nm),thus reducing the H₂O permeation rate. This is highly preferable inensuring excellent capacity of molecular sieves with selective CO₂permeance in the presence of water vapor. In fact, siliceous CHA filmswith a nominal Si/Al ratio of 100 (CHA_100) exhibited high CO₂/N₂ andCO₂/CH₄ separation performance, especially in the presence of watervapor (CO₂/N₂ separation factor (SF) of ˜13.4 and CO₂/CH₄ separationfactor (SF) of ˜37 at 30° C.). These SFs were higher than thecorresponding values (CO₂/CH₄ SFs of ˜5.2 and CO₂/CH₄ SFs of ˜31) underdry conditions. Such improvement could be ascribed to defect blocking byphysisorbed water molecules. Preferably, under wet conditions, thesiliceous, hydrophobic CHA membrane (CHA_100) could provide the high CO₂permeance of ˜1.1−1.4×10⁻⁷ mol·m⁻²·s⁻¹·Pa⁻¹, which was as high as ˜14 to19% of that through bare supports. Finally, the contribution ofmolecular transport through zeolitic and non-zeolitic parts wasquantitatively analyzed by combining information extracted from imageprocessing of fluorescence confocal optical microscopy images with a1-dimensional permeation model. The result showed that ˜19% and ˜20% ofthe total CO₂ permeance for CHA_100 were reduced due to transportinhibition by the physisorbed water molecules on the membrane surfaceand defects, respectively.

In the present invention, a secondary growth method was adopted for thefabrication of siliceous CHA membranes. First, a uniform CHA seed layer,which was mainly composed of plate-like CHA particles, was formed on aporous α-Al₂O₃ disc via a selective deposition method (Kim, E. et al.,Angew. Chem., Int. Ed. 2013, 52, 5280-5284). The seed layer was furtherhydrothermally inter-grown to form a continuous, siliceous CHA membrane.For secondary growth, three different nominal Si/Al ratios of 20, 100,and infinity in the synthetic precursor were used. This approach wasused to study the effect of the Si/Al ratio on the membrane propertiessystematically and, thus, the separation performances. Among the three,the most siliceous and thus most hydrophobic CHA membrane showed muchimproved separation performance for both CO₂/N₂ and CO₂/CH₄ mixturesunder wet conditions compared to that under dry conditions. In addition,fluorescence confocal optical microscopy (FCOM) revealed that the CHAmembranes showed distinctive features in the defect (crack) structuredepending on the Si/Al ratios in the synthetic precursors. Imageprocessing of the FCOM images provides correlation between the defectstructure and the separation performance quantitatively. Furthermore,this relationship was used to elucidate the effect of the hydrophobicityand defect structure of the CHA membrane on the CO₂ permselectivities inwet feeds. Finally, the CO₂ permselectivity of the CHA membranes in thepresence of H₂O vapor in the feed was compared with those of CHA andother types of zeolite membranes in the literature, and CHA membraneshave excellent performance under both dry and wet conditions were found.

In the present invention, it was found that the CHA zeolite membraneprepared by the method described above can provide and maintain a highCO₂ separation capacity even in a feed condition containing water,regardless of the temperature or pressure of the flue gas.

Accordingly, in another aspect, the present invention provides a CHAzeolite membrane produced by the method, wherein the CHA zeolitemembrane has a defect porosity of 0.1 to 1% and a z-axis tortuosity(tortuosity in a thickness direction of the defect) of 1 to 2.

In still another aspect, the present invention is directed to a methodof separating CO₂ from a mixture containing CO₂ and a molecule selectedfrom the group consisting of CH₄, N₂, O₂, C₂H₄, C₂H₆, C₃H₆ and C₃H₈using the CHA zeolite membrane.

In the present invention, the separation factor of CO₂/N₂ in the fluegas separation process containing water may be 10 to 30, and theseparation factor of CO₂/CH₄ in the upgrading process of biogascontaining water may be 10 to 80.

In the use of the CHA zeolite membrane according to the presentinvention, the separation of carbon dioxide includes all of separation,capturing and removal of carbon dioxide.

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to examples. However, it will be obvious to those skilled inthe art that these examples are provided only for illustration of thepresent invention and should not be construed as limiting the scope ofthe present invention.

Example Preparation Example 1: Fabrication of CHA Membrane

Synthesis of Seed Particle and Seed Deposition

All-silica CHA (Si-CHA) zeolite particles were synthesized following aliterature procedure (Diaz-Cabanas, M. J. et al., Synthesis andStructure of Pure SiO₂ Chabazite: The SiO₂ Polymorph with the LowestFramework Density. Chem. Commun. 1998, 1881-1882).N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH, 25 wt % in water,Sachem Inc.) was poured into distilled (DI) water in a Teflon beaker.Ethanol (anhydrous 200 proof, ≥99.5%, Sigma-Aldrich) was subsequentlyadded to the mixture, followed by the addition of tetraethylorthosilicate (TEOS 98%, Sigma-Aldrich), which served as the silicasource. The synthesis mixture was hydrolyzed overnight at roomtemperature and was further dried until the ratio of SiO₂ to waterreached ˜2.4. After that, the synthesis mixture became almost solid.Finally, an HF solution (48%, Sigma-Aldrich) was added dropwise to thesolid-like synthesis mixture. The final molar composition was 10 SiO₂: 5TMAdaOH: 5 HF: 30 H₂O. The resultant synthesis mixture was transferredto a Teflon liner in an autoclave for reaction. The autoclave containingthe synthesis mixture was placed on a rotating rack in a preheated ovenat 155° C., and after completing the reaction for 42 hours withrotation, the reaction was quenched with tap water. The resultingparticles were recovered by repeating centrifugation and washing fivetimes. In addition, the recovered particles were calcined at ˜600° C.for 12 hours at a ramp rate of 1° C·min⁻¹ under air flow (200 mL min⁻¹).

The seed deposition procedure has been reported elsewhere (Kim, E. etal., Uniform Si-CHA Zeolite Layers Formed by a SelectiveSonication-Assisted Deposition Method. Angew. Chem., Int. Ed. 2013, 52,5280-5284; Kim, E. et al., On the Synthesis and Characterization ofAll-silica CHA Zeolite Particles. Microporous Mesoporous Mat. 2014, 184,47-54). Briefly, —0.05 g of Si-CHA particles were added to ˜50 mL ofanhydrous toluene in a specially designed glass reactor with flowingargon (˜20 mL·min⁻¹). After that, an α-Al₂O₃ disc sandwiched between twocover glasses was fixed on a comb-shaped Teflon holder and placed in thesuspension inside the glass reactor. Detailed information about theplacement of the disc on the Teflon holder can be found in (Choi, J. etal., MFI Zeolite Membranes from a- and Randomly Oriented Monolayers.Adsorption 2006, 12, 339-360). About 50 mL of anhydrous toluene waspoured into the glass reactor under an argon environment. Subsequently,the glass reactor was sealed with Parafilm and ultra-sonicated (UC-10P,JEIO Tech, South Korea) for ˜20 min. The seeded α-Al₂O₃ disc wascalcined at 450° C. for 4 hours at a ramp rate of 1° C·min⁻¹.

FIG. 12 shows the scanning electron microscopy (SEM) images of theSi-CHA particles obtained by following the protocol described above. Asreported in previous studies by the present inventors (Kim, E. et al.,Uniform Si-CHA Zeolite Layers Formed by a Selective Sonication-AssistedDeposition Method. Angew. Chem., Int. Ed. 2013, 52, 5280-5284; Kim, E.et al., On the Synthesis and Characterization of All-silica CHA ZeoliteParticles. Microporous Mesoporous Mat. 2014, 184, 47-54), a majority ofcubic-like Si-CHA particles having a wide size distribution wereobserved, as well as a minor portion of plate-like Si-CHA particles(indicated by red arrows in FIG. 12 ). Despite being the minor portion,the plate-like Si-CHA particles were selectively deposited onto α-Al₂O₃discs to form a uniform seed layer by controlling the access to the discsurface following the above-mentioned approach reported in the previousstudy by the present inventors. Because of the broad size distribution,the large cubic-like Si-CHA particles were not appropriate for forming auniform seed layer, which is critical in forming a continuous membranevia the secondary growth methodology (Choi, J. et al., Uniformlya-Oriented MFI Zeolite Films by Secondary Growth. Angew. Chem., Int. Ed.2006, 45, 1154-1158; Lai, Z. et al., Microstructural Optimization of aZeolite Membrane for Organic Vapor Separation. Science 2003, 300,456-460). FIG. 12 c shows that the deposited plate-like CHA particleswere preferentially aligned. In particular, the X-ray diffraction (XRD)peak corresponding to the h0h-plane was pronounced, while the othersobserved in the powder XRD patterns almost vanished. The resulting seedlayer was comparable to that reported in the literature.

CHA Film Growth

Three types of CHA zeolite membranes were synthesized by varying theSi/Al ratio in the synthetic precursor. First, a certain amount ofTMAdaOH, Al(OH)₃ (for a finite Si/Al ratio), and CAB-O-SIL (CAB-O-Sil®M-5, Cabot Corporation) as a silica source were added to deionizedwater, and the mixture was further shaken using a shaking machine (LabCompanion, SI-300R, South Korea) overnight at room temperature. Afterthat, a certain amount of NaOH was added to the well-mixed syntheticprecursor, followed by shaking overnight at room temperature. The finalmolar compositions of the three synthetic precursors for secondarygrowth were 20 TMAdaOH: 100 SiO₂: 4,400 H₂O: 20 NaOH: x Al(OH)₃, wherex=0, 1, and 5 correspond to the nominal Si/Al ratios of infinity, 100,and 20, respectively. The prepared synthetic precursor was poured into aTeflon liner where a seeded α-Al₂O₃ disc had been placed vertically withthe help of a Teflon holder. The Teflon liner was placed in an autoclaveand the autoclave was moved to an oven preheated to 160° C. Thehydrothermal reaction for secondary growth was carried out at 160° C.for 6 days. After the reaction had finished, the autoclave was quenchedwith tap water. The as-synthesized membranes were recovered from theautoclave, washed with copious water, and dried at room temperature. Thedried membranes were further calcined at 550° C. for 12 hours at aheating ramp rate of 0.5° C.·min⁻¹ under air flow (200 mL·min⁻¹). Forconvenience, the calcined membrane samples are referred to as “CHA_x”,where x indicates the nominal Si/Al ratio in the synthetic precursor.

Example 1: Characterization of Membrane

SEM images of CHA particles, layers and separators were obtained using aHitachi S-4300. Prior to analysis, all samples were coated withPt-sputtering (Hitachi S-4300). The crystallinity of the Si-CHA zeolitesand orientation of the Si-CHA layers and separators were determinedthrough XRD (Rigaku Model D/Max-2,500 V/PC diffractometer, Japan) in theθ/2θ array using Cu Kα radiation (40 kV, 100 mA, λ=0.154 nm). Powder XRDpatterns of CHA zeolites were generated using the Mercury program(www.ccdc.cam.ac.uk). The CHA Zeolite Decision Information File (CIF)was downloaded from www.iza-online.org. The chemical content from themembrane surface to the portion of the α-Al₂O₃ support adjacent to theCHA film was analyzed by energy dispersive X-ray analysis (EDX, HitachiS-4800). This was used to evaluate the hydrophobicity of the separator.In addition, the contact angle of the water droplets on the membranesample at room temperature was measured using a light microscope(General Type Phoenix 300, Surface Electro Optics) and recorded overtime. To investigate the inner structures of the CHA membranes, FCOM(Zeiss LSM-700) was used after the membranes had been impregnated with afluorescent dye (fluorescein sodium salt, empirical formula:C₂₀H₁₀Na₂O₅, Sigma-Aldrich (Product No.: F6377)) having an approximatediameter of ˜1 nm. In the present invention, membrane staining wasperformed for 12, 24, and 96 hours. In addition, a simple screening testsuitable for excluding highly defective membranes was used. This testinvolves placing several droplets of 1 mM fluorescent dye solution onthe membrane surfaces and observing the dye distribution with time.

The Wicke-Kallenbach mode was adopted for measuring the permeation ratesof CO₂ and N₂ (or CH₄). The total pressures of both feed and permeatesides were maintained at 1 atm. In order to examine the effect of wateron the membrane, the feed gas was humidified. Specifically, the partialpressures of CO₂/N₂/H₂O (or CO₂/CH₄/H₂O) were 48.5 kPa/48.5 kPa/3 kPa,respectively. A total flow rate of 100 mL·min⁻¹ of CO₂ and N₂ (or CH₄)on a dry basis was fed to the membrane, and the CO₂/N₂ or CO₂/CH₄mixture permeated through the membrane. The permeate was carried by Hesweep gas (100 mL·min⁻¹) and was analyzed on-line by a gaschromatography (GC) system (YL 6100 GC for CO₂/N₂ and YL 6500 GC forCO₂/CH₄, Young Lin Instruments, South Korea) equipped with a packedcolumn (6 ft×⅛″ Porapak T for CO₂/N₂ and 30 m×0.320 mm GS-GasPro forCO₂/CH₄) and a thermal conductivity detector (TCD) for CO₂/N₂ analysisand a pulsed discharge ionization detector (PDD) for CO₂/CH₄ analysis.For reliable analysis, ˜5 mL·min⁻¹ of CH₄ for CO₂/N₂ and H₂ for CO₂/CH₄measurements were added to the swept permeate gas stream before reachingthe GC system. In addition, permeation tests with respect to thedifferent feed humidities and longer durations at 50° C. were conductedwith CHA_100 and co. In order to evaluate the effect of the humidity onthe separation performance of the membrane samples at 50° C., thepermeation tests were conducted at relative humidities of 0%, 26%, 60%,and 100% at 50° C. for 12 hours each. The relative humidities of 0%,26%, 60% and 100% at 50° C. correspond to water vapor partial pressuresof approximately 0, 3, 7 and 12 kPa, respectively. The long-termstability tests were conducted at 50° C. at the saturated water vapor of˜12 kPa. To expedite any deactivation, the membrane samples were heatedto 200° C. between the long-term measurements at 50° C. Specifically,the permeation tests were performed at 50° C. for 3 days, then at 200°C. for 2 days, and then again at 50° C. for 3 days.

Formation of CHA Membranes

FIGS. 1 a 1-c 1 and 1 a 2-c 2 reveal that the secondary growth of theseed layer (shown in FIG. S1b) led to well-intergrown, continuous filmsat SEM resolution for all three Si/Al ratio cases (20, 100, and ∞)considered in the present invention. CHA_20 and _∞ were composed ofgrains similar to the cubic-like CHA particles (FIG. 12 ). It appearsthat these grains were aligned in the diagonal direction along themembrane thickness. As a result, the protruding edges of the cubic-likeparticles led to rough surfaces in CHA_20 and _∞. On the other hand, thegrains on the top surface of CHA_100 were smooth and larger. Thethicknesses of CHA_20, _100, and _∞ were estimated to be 2.2, 9.2, and 2μm, respectively (FIG. 1 a 2-c 2). Further measurements provided averageand standard-deviation values for the membrane thickness (Table 1).However, the corresponding XRD patterns confirmed a random out-of-planeorientation (FIG. 1 a 3 and 1 c 3), indicating the failure of thepreferred alignment of the grains. In addition, the XRD pattern ofCHA_100 showed that it did not have a pronounced degree of preferredout-of-plane orientation, although the intensities of some XRD peaks,such as those corresponding to the (210) (201) and planes, were slightlyincreased relative to those in the calcined Si-CHA powder (FIG. 1 b 3).In particular, the increased intensity of the XRD peak corresponding tothe (210) (plane indicates that the 8 MR channels were positioned in theout-of-plane direction (that is, perpendicular to the zeolite surface)and the corresponding CO₂ permeance would be increased by a shorterpassage through the membrane.

TABLE 1 Sample Thickness (μm) CHA_20 1.6 ± 0.4 CHA_100 9.3 ± 0.9 CHA_∞2.3 ± 0.7

Example 2: CO₂ Permselectivities of CHA Membranes Example 2-1: CO₂/N₂Separation Performance Under Dry Conditions

Despite the continuity observed at SEM resolution, the CHA filmsynthesized with a nominal Si/Al ratio of 20 (CHA_20) showed very poorCO₂/N₂ separation performance under dry conditions. The maximum (max)CO₂/N₂ SF was estimated to be 2.2±0.2, which is relatively close to that(˜0.8) determined based on Knudsen diffusion (FIG. 2 a 1). In addition,the poor CO₂/N₂ separation performance of CHA_20 was similar to that ofthe CHA membranes reported in a previous study by the present inventors(Kim, E. et al., Chemical Vapor Deposition on Chabazite (CHA) ZeoliteMembranes for Effective Post-Combustion CO₂ Capture. Environ. Sci.Technol. 2014, 48, 14828-14836). The low maximum CO₂/N₂ SF of 2.2 wasmuch smaller than the expected value of ˜10-20 through the CHAmembranes, indicating the presence of pronounced, unwanted non-zeoliticparts in CHA_20 (Kim, E. et al., Chemical Vapor Deposition on Chabazite(CHA) Zeolite Membranes for Effective Post-Combustion CO₂ Capture.Environ. Sci. Technol. 2014, 48, 14828-14836; Krishna, R. et al.,Segregation Effects in Adsorption of CO₂-Containing Mixtures and TheirConsequences for Separation Selectivities in Cage-Type Zeolites. Sep.Purif. Technol. 2008, 61, 414-423). Compared to CHA_20, CHA_100 and _∞showed higher maximum CO₂/N₂ SFs of ˜5.2±0.1 and ˜7.3±2.0, respectively(FIG. 2 b 1-c 1). It was noted that the CO₂/N₂ SFs monotonicallydecreased with temperature, similar to the trend of the previouslyreported SSZ-13 membranes with the maximum CO₂/N₂ SF being ˜10-11 at˜20-25° C. Among the three membranes in the present invention, CHA_∞showed the highest maximum CO₂/N₂ SF at 30° C. under dry conditions.This suggests that a small amount of Al in the secondary growth solutionis beneficial for obtaining a less-defective membrane structure. Themonotonic increase in the separation performance with the increase inthe Si/Al ratio in the synthetic precursor is similar to that observedin a previous study. However, in that paper, it was reported that theuse of a small amount of Al in the synthetic precursor (for example,Si/Al ratio of 125) led to the formation of undesired AFI zeolitemembranes. However, despite the absence of Al, the synthesis ofhigh-performance CHA membranes was still possible (CHA_∞ in FIG. 2 c 1).To the best of common knowledge, the secondary growth of the CHA seedlayer with a nominal Si/Al ratio of infinity in the synthetic precursoris the first example of the fabrication of a high-performance CHAmembrane (here, CHA_∞).

In addition, the CO₂ permeance through CHA_100 was higher than thatthrough CHA_∞ (FIG. 2 b 1-c 1), even though the corresponding membrane(˜9.2 μm) was much thicker than CHA_∞ (˜2 μm) (FIG. 1 b 2-c 2). Becausethe permeance across a membrane is generally reciprocal to the membranethickness, such behavior indicates a beneficial contribution of othermembrane properties. Among the possible factors, the good CO₂ permeanceis due to the preferred alignment of the 8 MR channels in CHA_100 (FIG.1 b 3). As mentioned above, the XRD peak arising from the (210) plane,which is perpendicular to the 8 MR channel in the [100] axis, waspronounced. Furthermore, the intensity of the XRD peak corresponding tothe (003) plane, which is perpendicular to the 6 MR pore in the [001]axis, was reduced. This indicates that the 8 MR channel was more alignedalong the membrane thickness, while the mass transfer inhibition by the6 MR pores was significantly weakened. It seems that the preferredalignment of the 8 MR channels helped to increase the CO₂ permeance,despite the greater thickness of CHA_100. Nevertheless, at this point,it is impossible to exclude the possible contribution of moleculartransport through non-zeolitic defects to the higher CO₂ permeance, andthis will be discussed in detail later.

Previously, Si-CHA zeolite seed particles have been synthesized in thepresence of hydrofluoric acid (HF), where F⁻ served as an anioniccounterpart with respect to the cationic structure directing agent (SDA,here TMAda⁺). However, the direct adoption of the synthetic protocol forthe secondary growth of the CHA seed layer would not work, apparentlydue to the dissolution of the pre-deposited CHA seed particles by HF.Recently, the use of TMAdaF as a SDA (which can be considered to havethe combined role of TMAdaOH and HF) instead of the conventional TMAdaOHSDA has been shown to be effective for the fabrication of CHA membranesvia secondary growth. Nevertheless, the use of TMAdaOH is highlydesirable for manufacturing high-performance CHA membranes, because itdoes not require an additional, sophisticated procedure to obtainTMAdaF.

Example 2.2: CO₂/N₂ Separation Performance Under Wet Conditions

Considering the third main component, H₂O vapor, in the CO₂-containingstreams, the CO₂ permselectivities of the three CHA membranes under wetconditions are investigated. Specifically, CHA_20, which showed poorCO₂/N₂ separation performance (maximum SF of ˜2.2 at 30° C.) under dryconditions, also exhibited a low CO₂/N₂ separation performance (maximumSF of ˜1.6 at 100° C.) under wet conditions (FIG. 2 a 2), confirming thepronounced presence of undesired non-zeolitic defects. The increase inthe nominal Si/Al ratio of the secondary growth precursor from 20 to 100led to an improvement in the maximum CO₂/N₂ SF from ˜5.2±0.1 under dryconditions to ˜13.4 ±1.8 under wet conditions at the same temperature,30° C. (CHA_100 in FIG. 2 b 1-b 2). This trend was also observed forCHA_∞ (FIG. 2 c 1-c 2). However, a difference was found; although theCO₂/N₂ SF of CHA_100 monotonically decreased with temperature (FIG. 2 b2), that of CHA_∞ showed a maximum CO₂/N₂ SFs of ˜15 in the temperaturerange of 50-75° C., not 30° C., under wet conditions (FIG. 2 c 2).Considering the representative temperature (˜50-75° C.) of flue gas, thecomparable CO₂/N₂ SFs of ˜15 at ˜50-75° C. through CHA_∞ is highlydesirable for effective membrane-based carbon capture. Notably, despitethe use of the same seed layer, the appropriate choice of Si/Al ratio inthe secondary growth precursor was critical for obtaininghigh-performance CO₂-permselective CHA membranes. Moreover, the feedcomposition of the simulated post-combustion flue gas (here, at a CO₂partial pressure of ˜15 kPa) was considered to investigate theseparation performance of CHA_100 and _∞ in the presence of H₂O in thefeed (FIG. 13 ). The results are comparable with those observed forequimolar CO₂/N₂ binary feeds (FIG. 2 b 2-c 2). This result could beascribed to the almost linear adsorption behavior of both CO₂ and N₂molecules inside the CHA zeolites.

Despite previous results concerning water-assisted CO₂ permselectivity,the monotonic decrease in the CO₂/N₂ SF of the SSZ-13 membrane withincreased temperature makes it undesirable for practical use. Inparticular, considering the plausible temperature fluctuations of fluegas streams around the representative temperature (˜50° C.),establishment of maximum separation performance in the temperature rangeof ˜50-75° C. is highly desirable. In this respect, CHA_∞ prepared inthe present invention, which had a CO₂/N₂ SF greater than 10 in thepresence of H₂O vapor in a temperature range of ˜50-75° C., meets theabove-mentioned requirement for practical use.

The improved CO₂/N₂ separation performance under wet conditions can beascribed to the hydrophobic surface of the CHA membranes. Because of thesmaller size of the H₂O molecule (0.265 nm), preventing the entry of H₂Ointo the CHA zeolite pores is not a sound approach for minimizing theH₂O permeation rate. Instead, the control of the hydrophobicity of thesurface of the CHA membranes is effective for reducing the adsorption ofH₂O and, thus, its permeation rate. The EDX analysis shown in FIG. 1 a2-c 2 reveals that there is a siliceous component near the surface inall three CHA membranes. Considering the trend in the distribution of Alatoms along the membrane thickness, it appears that CHA_20 had a higherportion of Al atoms compared to CHA_∞. It was noted that the proportionof Si atoms approached zero after passing the interface in CHA_∞,whereas that in CHA_20 did not reach zero, suggesting the possibleexistence of CHA zeolites deep inside the α-Al₂O₃ disc support. Incontrast, a higher siliceous portion was observed throughout themembrane thickness for CHA_100. Accordingly, CHA_100 could be consideredto be the most hydrophobic membrane of the three prepared membranes.

To clarify the hydrophobicities of the CHA membranes, the contact anglesof water droplets on the membrane surfaces are measured (FIG. 14 ). Thewater droplet on CHA_20 was immediately infiltrated into the membranesample, apparently into the non-zeolitic defects, thus failing areliable test. CHA_∞, which clearly contained a lower Al portion thanCHA_20 (FIG. 1 a 2 and 1 c 2), had a water droplet contact angle of˜25°. This contact angle indicates the lower hydrophobicity of CHA_∞,especially compared to CHA_100 (contact angle of ˜80°). Unlike highlydefective CHA_20, CHA_100 and _∞ both held the water droplet for up toduration of 5 minutes (FIG. 14 ), suggesting narrower or fewer defectsin these two membranes. Both the EDX and contact angle measurements ledus to the conclusion that the surface of CHA_100 was most hydrophobic,followed by CHA_∞, while the surface of CHA_20 was not continuous,possibly because of the insufficient intergrowth during secondarygrowth.

It was noted that the dry CO₂/N₂ SF of CHA_∞ was higher that of CHA_100(FIG. 2 b 1-c 1), possibly indicating a lower degree of defects inCHA_∞. Despite the expected lower degree of defects in CHA_∞, its wetCO₂/N₂ separation performance was inferior to that of CHA_100, whichshowed higher CO₂ permeance and a marked CO₂/N₂ SF at ˜30° C. (FIG. 2 b2-c 2). Because CHA_100 was the most hydrophobic membrane (FIGS. 1 a 2and 14), the degree of CO₂ permeance reduction arising from the presenceof H₂O vapor was much lower than that for CHA_∞ (FIG. 2 b 1-b 2 and 2 c1-c 2). Despite the lower CO₂ permeance, CHA_∞ exhibited significantCO₂/N₂ SFs (˜9.7-15.2) throughout ˜30-75° C. (FIG. 2 c 2). This steadyCO₂/N₂ SF at ˜30-75° C. indicates robustness with respect to theplausible temperature-fluctuation of the flue gas or biogas stream of˜50° C. in the presence of H₂O vapor. Considering that the final CO₂/N₂separation performance of CHA membranes with some defects is a coupledfunction of (1) the non-zeolitic defects associated with thephysisorption of H₂O vapor and (2) intrinsic zeolitic parts, the CO₂/N₂separation performance shown in FIG. 2 b 2-c 2 needs to be related tothe defect structure.

The good separation performance of CHA membranes under wet conditions(FIG. 2 c 2) is not expected to deteriorate over time at temperaturesbelow 100° C., mainly because of the robust CHA zeolite structure. Inorder to confirm this expectation, the less hydrophobic CHA_∞ was testedfirst, because this is more likely to show deactivation if any occurs.The response of CHA_∞ with respect to the simulated CO₂/N₂ feed (a molarcomposition of 15:85 on a dry basis) at ˜50° C. with relative humiditiesof 0%, 26%, 60% and 100% was investigated (FIG. 3 a ). An enhancement inthe CO₂ permselectivity (equivalent to that observed at 50° C. in FIG.13 b ) caused by water vapor was observed immediately after the additionof water vapor (from 0% to 26%) and was maintained. Furthermore, theaddition of more water vapor to relative humidities of 60% and 100%reduced the permeances of both CO₂ and N₂ molecules to the same extent,so that the corresponding CO₂/N₂ SF remained constant at about 27. Inaddition, the rigorous long-term stability test of CHA_∞ in thesimulated CO₂/N₂ feed at 50° C. with a relative humidity of 100%confirmed its robustness (FIG. 3 b ). Even after its exposure to harsherfeed conditions (at 200° C. for 2 days), the CO₂/N₂ separationperformance on reaching the target temperature of 50° C. was fullyrecovered and maintained for an additional 3 days (FIG. 3 b ), revealingthe robustness of the CHA zeolite inorganic membranes. In addition, themore hydrophobic CHA_100 showed an improved and steady CO₂/N₂ SFimmediately after adding water vapor to the dry feed (FIG. 3 c ).Desirably, such CO_(2/)N₂ SF was well preserved. Further addition ofwater vapor (relative humidities of 60% and 100%) selectively reducedthe permeance of N₂ molecules and, accordingly, increased the CO₂/N₂ SFmonotonically.

The long-term stability test of CHA_100 in the simulated CO₂/N₂ feed at50° C. with a relative humidity of 100% (FIG. 3 d ) also confirmed itshigh suitability for practical use (that is, low CO₂ partial pressuresand under wet conditions). Apparently, the harsh treatment at 200° C.for 2 days did not damage the membrane performance In addition, theCO₂/N₂ separation performance before the treatment was recovered welland maintained for an additional 3 days (FIG. 3 d ).

Example 3: CO₂/CH₄ Separation Performance of CHA Membrane

In addition to the CO₂/N₂ separation performance, the CO₂/CH₄ separationperformance of CHA_100 and _∞ was also measured (FIG. 4 ). As expectedfrom their high CO₂/N₂ biogas separation performance (FIG. 2 ), bothCHA_100 and _∞ showed good CO₂/CH₄ separation performance under dryconditions, having corresponding maximum CO₂/CH₄ SFs as high as ˜31 and˜61, respectively, at 30° C. (FIGS. 4 a and 4 c ). The observation ofhigher CO₂/CH₄ SFs than CO₂/N₂ SFs was expected, because the diffusionalbehavior inside the microporous channels is a function of molecular size(CH₄; 0.38 nm vs. N₂; 0.364 nm). Furthermore, the CO₂/CH₄ permeationbehaviors of both CHA_100 and _∞ under wet conditions (FIGS. 4 b and 4 d) exhibited a similar trend to that of the CO₂/N₂ separation performanceunder wet conditions (FIG. 2 b 2-c 2). That is, the CO₂ permeancedecreased, but the permeance of the larger CH₄ molecule was moresignificantly decreased at 30° C. It was noted that the permeance of CH₄was much lower than that of N₂, apparently due to the significantinhibition of the larger CH₄ molecule by the physisorbed watermolecules. Nevertheless, the wet CO₂/CH₄ SFs at 30° C. did not increaseas much for CHA_100 and decreased for CHA_∞ [(1) dry SF of 30.7 vs. wetSF of 37.2 for CHA_100 and (2) dry SF of 60.8 vs. wet SF of 16.5 forCHA_∞]. Compared to that in the CO₂/N₂ binary mixture, the CO₂ permeancereduction was more significant in the CO₂/CH₄ binary mixture, as thepresence of the larger CH₄ molecule was likely to disfavor the transportof CO₂. Nevertheless, it was found that for CHA_100, the reduction inCO₂ permeance was not considerable, whereas that for CHA_∞ wassignificantly decreased. This can be ascribed to the surfacehydrophobicity/hydrophilicity, as reflected by the higher and lowercontact angles of water droplets of 80° for CHA_100 and 25° for CHA_∞ ,respectively (FIG. 14 ). Considering the fact that the temperature ofbiogas streams is ˜40-70° C. and they contain ˜3-6% H₂O vapor, CHA_100and _∞, which showed wet CO₂/CH₄ SFs of 35.6 and 27.4, respectively, at50° C., are suitable for the separation of CO₂ molecules from biogasstreams.

At this point, the monotonically decreasing trend of CO₂/N₂ and CO₂/CH₄SFs with an increase in temperature is not desirable for reliable use.Maximum performance should be achieved at ˜50° C., a representative fluegas and biogas temperature, and should be insensitive to small changesin temperature. The low Si/Al ratio in the synthetic precursor couldcontribute to the formation of more defects in the zeolite membranes (asobserved in CHA_20 in the present invention), possibly because of therelatively high polarity and repulsion of the growing zeolite grains.Concerning CO₂/N₂ and CO₂/CH₄ separation through SSZ-13 membranes underwet conditions, the permeances of CO₂, N₂, and CH₄ all decreased attemperatures below 100° C., but the corresponding SFs were increasedbecause of the greater inhibition of the slowly permeating, larger N₂and CH₄ molecules by H₂O. The Si/Al ratio in the synthetic precursorsignificantly affects the hydrophobicity of the final membrane surfaceand constituents, whereas defect formation is a sensitive function ofthermal processing. As both factors influence the final performance, theidentification of defects and the elucidation of their effects on theperformance are also important for understanding and achieving high CO₂permselectivities in realistic feeds (here, water-vapor-containing CO₂streams).

Example 4: Investigation of Defects in CHA Membranes through FCOM

In addition to the measurement of CO₂ permselectivities of CHA_20, _100,and _∞, the defect structures were imaged via FCOM to understand therelationship between the structures and separation performance FCOManalysis is an attractive technique for studying the structure ofdefects embedded in a membrane without damaging the membrane structure.FIG. 6 reveals that despite the significant discrepancy in theseparation performance of CHA_20 and _∞ (FIG. 2 a 1-a 2 and 2 c 1-c 2),the cracks observed in the FCOM images were similar. Specifically, thecross-sectional view FCOM images show that the cracks, which constituteda defect network, as shown in the top-view FCOM images, propagated allthe way down to the interface between the CHA membrane and the α-Al₂O₃disc. Although it is challenging to determine the contribution of thecracks to the permeation rates in both CHA_20 and _∞ because of thelimited optical resolution, the permeation results in both dry and wetconditions shown in FIG. 2 a 1-a 2 and 2 c 1-c 2 confirm that CHA_20 wasmore defective than CHA_∞. Thus, the sizes or widths of the cracks inCHA_20 were apparently larger than those in CHA_∞. Despite theabove-mentioned limited resolution, the FCOM images of the thickerCHA_100 membrane indicate that although the cracks also penetrated tothe interface, the number of cracks was much larger than those in theother two membranes.

Considering that the dyeing process is kinetically relevant, the dyeingduration was systematically varied from 12 hours through 24 hours to 96hours (FIGS. 7 to 8 ). This approach is beneficial for obtaining afundamental understanding of the defect structure. FIGS. 7 to 8 revealthat the degree of crack dyeing in CHA_20 and _∞ with respect to thedyeing duration was similar. The cracks, which propagated from themembrane surface to the interface, were accessed by the dye moleculesafter 12 hours (FIGS. 7 a 2-8 a 2 and 7 c 2-8 c 2) and saturated after24 hours (FIGS. 7 a 3-8 a 3 and 7 c 3-8 c 3). However, dyeing for 96hours resulted in a marked difference, while it was almost impossible todistinguish the cracks in CHA_20 because of the extremely highconcentration of dye molecules (FIGS. 7 a 4 and 8 a 4), additionalcracks were observed for CHA_∞, but these ended in the middle section(indicated by the yellow arrows in FIGS. 7 c 4 and 8 c 4). For bettercomparison, additional FCOM images are shown in FIGS. 15 and 16 .Compared to those of CHA_20 and _∞, the cracks in CHA_100 were slowlydyed with an increase in dyeing duration (FIGS. 7 b 2-b 4 and 8 b 2-b4). Unless the thickness of CHA_100 is similar to those of CHA_20 and_∞, a fair evaluation of the size and number of defects is challenging.Nevertheless, it is reasonable to conjecture that the size of thedefects in CHA_100 was comparable to or smaller than that of CHA_∞,whereas the number of defects or defect density of CHA_100 was higherthan that of CHA_∞.

Although the FCOM images provide detailed information about the defectstructure, the dyeing process required for FCOM characterizations alsocan reveal the degree of undesired defects. The inset in FIG. 7 showsthat the initially transparent water on the permeate side of CHA_20became colored after 12 hours and that the concentration of dye on thepermeate side gradually increased over time up to 96 hours. This wasinterpreted as a result of the steady passage of dye molecules along thedefective cracks. Thus, the size and/or number of defects in CHA_20 weresignificantly greater than those of the other membranes (CHA_100 and_∞).

Combining the FCOM characterization (defect structure inside the CHAmembranes) with the water contact angle results (hydrophilicity of thesurface on the CHA membranes), a simple and rapid but reliableevaluation tool that can determine the success of a membrane fabricationwas developed. Specifically, a dye solution, which was originallyallowed to diffuse into the defect structure in the CHA membranes forFCOM characterization, was dropped onto the membrane surface and itsdiffusion into the membrane was tracked with time (FIG. 17 ).Technically, this tool can be considered as a time-resolvedvisualization of the water contact angle measurement. Considering thatoptical characterization is not suitable for the selectivecharacterization of a zeolite membrane having a thickness on the orderof 100 nm, simple observation of the diffusional behavior of the droppeddye molecules can provide a useful insight into the formation ofundesired structures. First, this simple test (FIG. 17 a ) reveals thatall of the dye molecules on CHA_20 penetrated the defects immediatelyafter dropping. This result was consistent with the FCOM images (FIGS. 7to 8 ) and water contact angle results (FIG. 3 a ). Thesecharacterizations indicate that CHA_20 was the most defective membrane.Secondly, the surface of CHA_100 was more hydrophobic than that of CHA_∞(FIG. 17 b-c ), as already indicated by their respective water contactangles (CHA_100: 80° vs. CHA_∞: 25° as shown in FIG. 3 a ). However,this test does not allow for the differentiation of differences in thedefect structures, in contrast to FCOM results. In short, this simpletest is thought to be proposed for the first time in the presentinvention and can be used as a quick screening process to aid in theexclusion of highly defective membranes.

Example 5: Elucidation of CO₂ Separation Performance Based onQuantitative Membrane Properties

To date, the qualitative analysis of defects in zeolite membranes basedon the FCOM technique has mainly focused on identifying the defectdensity and distribution and correlating these factors with the finalseparation performance However, quantitative information about the sizeand number of defects and association with the final membraneperformance would be useful. Thus, a quantitative analysis of defects inCHA_20, _100 and _∞ was conducted based on an approach reported in aprevious study by the present inventors. The resulting processed defectsare schematically illustrated in FIG. 18 . In the case of CHA_100, thedifferentiation of bright and dark spots in the FCOM image was notpossible because of the presence of broad, extremely bright spots (FIG.6 b 1-b 3). Therefore, the image processing was performed except for thevery bright spot. The quantitative properties extracted from the FCOMimages were complemented with a 1-D permeation model, which was used toestimate the representative properties (tortuosity and porosity) of thedefects. The relevant properties are summarized in Table 2.

FIG. 9 graphically summarizes the defect size and porosity and thecontribution of the defects to the final molar flux of the fasterpermeating component (i.e., CO₂). Specifically, the defect sizes forCHA_20, _100 and _∞ were estimated to be 2.8, 3.1 and 1.3 nm,respectively, and the corresponding porosities were 0.19%, 0.30% and0.08% and 0.13%, respectively. As mentioned above, it appears that thedistributions of defects in CHA_20 and _∞ were comparable to each other(FIGS. 6 a 1-a 3 and 6 c 1-c 3), and accordingly, their pixel-based areafractions were similar (Table 2). However, a rigorous analysis based ona combination of image processing and permeation modeling reveals thatthe defect size (˜2.8 nm) in CHA_20 was ˜2 times of that (1.3 nm) inCHA_∞. In turn, this fact confirms the lower CO₂ permselectivity ofCHA_20 under dry conditions (FIG. 2 a 1) than that of CHA_∞ (FIG. 2 c1). Furthermore, the CO₂ molar fluxes passing through the defects inCHA_20 and _∞ at 30° C. under dry conditions accounted for 27% and 5% ofthe total CO₂ molar flux through the membranes, respectively (FIG. 9 ).Surprisingly, it was noted that despite the porosity of 0.19%, the molarflux of CO₂ through the defects in CHA_20 was pronounced, indicating theimportance of synthetic (here, nominal Si/Al ratio) or post-treatmentmethods for the effective reduction of defects in CHA membranes. Indeed,the reduction in porosity from 0.19% to 0.08% could decrease the molarflux through the defects from 27% to 5% (FIG. 9 ). This is consistentwith the previous conclusion showing that a very low number of defects(less than 1%) significantly degrade the zeolite membrane performance Infact, the difference in the defect sizes (2.8 nm for CHA_20 vs. 1.3 nmfor CHA_∞) was key to the substantial change in their CO₂permselectivities (FIG. 2 a 1-2 c 1).

TABLE 2 Tortuosity x- y- z- Area Porosity Sample direction directiondirection fraction^(a) (%)^(b) Ref. SSZ-13 2.17 2.23 1.10 6.85 × 10⁻²0.27 1 Membrane CHA_20 3.02 3.10 1.35 5.36 × 10⁻² 0.24 Present inventionCHA_100 2.55 2.56 1.58 8.17 × 10⁻² 0.30 Present invention CHA_∞ 3.813.69 1.24 4.86 × 10⁻² 0.13 Present invention

As already inferred from the CO₂ separation performance in FIG. 2 b 1-c1, the more defective CHA_100 had a lower maximum CO₂/N₂ SF (5.2) underdry conditions (for convenience, referred to as dry maximum CO₂/N₂ SF)than that of CHA_∞ (7.3). Thus, the visualized defect structure alongwith the hydrophobicity (as indicated by the water droplet contact angle(FIG. 14 b-c ) and EDX results (FIG. 1 b 2-c 2)) could account for thepermeation behavior (FIG. 2 b 2-c 2) of both membranes under wetconditions. The reduction in CO₂ permeance under wet conditions throughCHA_∞ (less hydrophobic) seemingly resulted from the combined adsorptionof water vapor in the defects and on the membrane surface. Thispreserved the high molecular sieving ability for CO₂ (FIG. 2 c 2). Incontrast, CHA_100, which was more hydrophobic and had a larger number ofdefects, provided high CO₂ permeance because of the high hydrophobicitythereof. Although some water vapor was adsorbed on the defects, the CO₂molecules could pass through the preferentially out-of-plane orientedCHA zeolite channels.

Compared to that under dry conditions, the membrane separationperformance under wet conditions is a strongly couple function of thezeolite structure, surface hydrophobicity/hydrophilicity, andsize/number of defects. Thus, summarized all possible membraneproperties relevant to the permeation behavior of CHA_100 and _∞ andtheir CO₂/N₂ permeation results at 30° C. are summarized in Table 3, inwhich those of another SSZ-13 membrane (for convenience, referred to asSSZ-13 in this section) are also given for comparison. First, theeffects of defects (defect (crack) porosity and defect size) on theCO₂/N₂ SFs of CHA_∞ and SSZ-13 were considered. The porosity differencebetween the two membranes (0.08% for CHA_∞ and 0.15% for SSZ-13 in Table2) had a significant effect on their respective dry maximum CO₂/N₂ SFsof 7.3 and 4.6. This observation is consistent with the relationshipbetween the porosity and dry maximum CO₂/N₂ SF for CHA_20 and _∞. Morespecifically, given the similar area fraction of CHA_20 and _∞ (Table3), the crack size was critical in determining the final separationperformance. Thus, the crack size of CHA_100, which could not bedetermined from its FCOM images, will be less than ˜3.1 nm to achievethe dry maximum CO₂/N₂ SF of 5.2, because the corresponding crackdensity was higher than that of CHA_∞ and SSZ-13.

TABLE 3 Dry Wet CO₂ CO₂ 100- Wet/ Water permeance × Dry permeance × WetWet/Dry Wet/Dry Dry Thick- contact Defect 10⁷ CO₂/ 10⁷ CO₂/ CO₂ CO₂ CO₂/Membrane ness angle size Defect (mol · m⁻² · N₂ (mol · m⁻² · N₂permeance permeance N₂ sample (μm) (°) (nm) density s⁻¹ · Pa⁻¹) SF s⁻¹ ·Pa⁻¹) SF (%) (%) SF Ref. SSZ-13 ~3.5 78 1.8 Middle 1.4 4.6 0.37 10 26 742.2 Hong S. et al.1) CHA_100 ~9.3 80 3.1 Relatively 2.3 5.2 1.4 13.4 6139 2.6 Preparation High Example 1 CHA_∞ ~2.3 25 1.3 Middle 1.5 7.3 0.379.7 25 75 1.3 Preparation Example 1 1) Hong, S. et al., Healing ofMicrodefects in SSZ-13 Membranes via Filling with Dye Molecules and ItsEffect on Dry and Wet CO₂ Separations. Chem. Mater. 2018, 30, 3346-3358.

For a fair comparison, the permeation behavior caused by the change fromdry to wet conditions was focused. Specifically, the CO₂ permeance (wetCO₂ permeance) and CO₂/N₂ SF 30° C. (wet CO₂/N₂ SF) under wet conditionswere compared with those (dry CO₂ permeance and dry CO₂/N₂ SF) under dryconditions (Table 3); the respective ratios are referred to as wet/dryCO₂ permeance and wet/dry CO₂/N₂ SF, respectively.

For CHA_∞, it may be reasonably speculated that water vapor was adsorbedin the defects and further adsorbed on the membrane surface of the lesshydrophobic CHA_∞. Assuming that all of the defects were occupied andblocked by water vapor, for CHA_∞, the reduction in CO₂ permeation(i.e., wet/dry CO₂ permeance) was caused by defect blocking (˜5%; FIG. 9) and surface inhibition (—70%). In a similar manner, for SSZ-13, thecontributions to the reduction in CO₂ permeance arising from defectblocking and surface inhibition were ˜11% and ˜64%, respectively. Inaddition, the degrees of reduction due to defect blocking and surfaceinhibition for CHA_100 were estimated to be 9% and 30%, respectively. Itwas noted that the less degree of surface inhibition due to thephysisorbed water molecules was observed for hydrophobic SSZ-13 (˜64%)and CHA_100 (˜30%) against less hydrophobic CHA_∞ (˜70%). The relativeportions of CO₂ permeances due to defect blocking and surface inhibitionalong with those of wet CO₂ permeances are displayed in FIG. 10 .

Subsequently, the values of the wet/dry CO₂/N₂ SF for SSZ-13, CHA_100,and CHA_∞ are 2.2, 2.6, and 1.3, respectively. It appears that thewater-blocking defects in SSZ-13 and CHA_100 contributed to increasingtheir molecular sieving abilities, as indicated by the higher wet/dryCO₂/N₂ SFs of 2.2 and 2.6, respectively. However, CHA_∞, which had thelowest degree of defects as indicated by the highest dry maximum CO₂/N₂SF of 7.3, was unlikely to gain a marked benefit from the blocking ofdefects with water molecules, because the defect effects were much lesssignificant than those of the other two membranes. Instead, asignificant loss in CO₂ permeance without any improvement of CO₂/N₂ SFsoccurred for less hydrophobic CHA_∞ by surface inhibition (up to ˜56%)because of the physisorbed water molecules.

The absolute values of the CO₂ permeances obtained through the zeoliticand non-zeolitic parts were investigated (FIG. 10 ). First, the relativeportion of molecular transport through the cracks in CHA_100 (˜9%) wascomparable to that in CHA_∞ (˜5%). However, the corresponding CO₂permeance (˜2.0×10⁻⁸ mol·m⁻²·s⁻¹·Pa⁻¹) was almost 2.6 times of that(˜7.7×10⁻⁸ mol·m⁻²·s⁻¹·Pa⁻¹) in CHA_∞ (FIG. 10 ). Then, thewater-blocking of higher-density defects in CHA_100 (FIG. 6 b ) couldeffectively help to achieve higher molecular sieving in favor of CO₂under wet conditions (FIG. 2 b 2-c 2). Second, in less hydrophobicCHA_∞, the CO₂ permeance (˜1.0×10⁻⁷ mol·⁻²·s⁻¹·Pa⁻¹) reduced by thesurface inhibition was much higher than those (˜7.0×10⁻⁸mol·m⁻²·s⁻¹·Pa⁻¹) for CHA_100 and SSZ-13 (FIG. 10 ). Finally, the CO₂permeance due solely to the zeolite parts for both CHA_100 and _∞ wasestimated. For this purpose, the CO₂ permeance due to thedefect-blocking was subtracted from the dry CO₂ permeance. Specifically,it was estimated to be as high as 2.1×10⁻⁷ mol·m⁻²·s⁻¹·Pa⁻¹ for CHA_100,and this value was higher than the expected value of 1.4×10⁻⁷mol·m⁻²·s⁻¹·Pa⁻¹ in CHA_∞. This strongly indicates a beneficial role ofthe aforementioned preferred out-of-plane orientation in CHA_100.

Example 6: Comparison in Separation Performance between CHA 100 and ∞

FIG. 11 a-b shows a summary of the CO₂/N₂ and CO₂/CH₄ separationperformance of CHA_100 and _∞, as well as those of other zeolitemembranes. Only the CO₂ separation performance measured at ˜40-50° C.under dry and wet conditions is shown in FIG. 11 . As already shown inFIG. 2 c 1-c 2, the CO₂/N₂ SFs of CHA_∞ for the wet equimolar CO₂/N₂mixture were increased compared to those under dry conditions. Thistrend had been also found for other hydrophobic zeolite membranes (DDRand SSZ-13). However, hydrophilic NaY membranes (Si/Al ratio of ˜1.5-3)showed the opposite trend. Although these membranes showed a markedCO₂/N₂ separation performance under dry conditions, they lost thisability completely under wet conditions (water vapor pressure of ˜2.6kPa). This indicates the importance of hydrophobicity in membranes forsecuring high performance under wet conditions, as has been addressedand emphasized above. In addition, CHA_∞ was not significantlyhydrophobic (as reflected by the lower water droplet contact angle; 25°vs. 80° for CHA_100, as shown in FIG. 14 ). Accordingly, the CO₂permeance for CHA_∞ decreased more under wet conditions. Despite themuch lowered CO₂ permeance in CHA_∞, some defects, where water vapor wasapparently condensed, were beneficial for securing a high CO₂/N₂ SFunder wet conditions. Nevertheless, the optimal defect size should besatisfied because the wider defects (here, ˜3.5 nm for CHA_20, as shownin FIG. 9 ) negated the separation performance regardless of thepresence of water vapor in the feed (FIG. 2 a 1-a 2). The same trend asthat observed for permeation for CHA_100 and _∞ in dry and wet CO₂/N₂feeds (FIG. 11 a ) was also observed for CO₂/CH₄ separation (FIG. 11 b), with a minor difference being the comparable CO₂/CH₄ SFs under dryand wet conditions. This trend can be explained by the good dry CO₂/CH₄performance through the 8 MR zeolite membranes. Although a negativeeffect of non-zeolitic defects on the final performance could not beexcluded completely, the defects present in CHA_100 and _∞ did notprovide a non-selective pathway for the larger CH₄ molecules under dryconditions. Under wet conditions, water molecules physisorbed in thedefects would not contribute to a significant reduction in the CH₄permeance. Instead, the transport of the faster permeating species, CO₂,was inhibited. This reduction in the CO₂ permeance was still much higherfor the less hydrophobic CHA_∞. Considering the fact that a morehydrophobic DDR membrane (water contact angle of 95°) showed a similarpattern to that of CHA_∞ (FIG. 11 b ), it appears that an appropriatecombination of hydrophobicity and defect density/size is required toachieve high CO₂/CH₄ separation performance under wet conditions.Nevertheless, minimization of defects is always desirable for securinghigh performance under dry conditions. In particular, CHA_100 showed thehighest molar flux of CO₂ in both CO₂/N₂ and CO₂/CH₄ binary mixtures,having comparable CO₂ permselectivities under wet conditions (FIG. 11 ).Specifically, the CO₂ permeances at 50° C. under wet conditions were ashigh as ˜20-25% of the CO₂ permeance through bare supports with ˜150 nmpore size. Finally, although the difference in the kinetic diameters ofCH₄ (0.38 nm) and N₂ (0.364 nm) was extremely small, the resultingperformance in favor of CO₂ molecules was quite distinct under both dryand wet conditions.

According to the present invention, a uniform CHA seed layer wasprepared via the physical attachment of plate-like Si-CHA seed particlesto an α-Al₂O₃ disc. The limited accessibility for seed particles toreach the surface of the α-Al₂O₃ disc was key to obtaining a uniformseed layer. Despite being the minor portion, the plate-like particlesdominated the seed layer. The subsequent secondary growth of the seedlayer with a synthetic precursor that had nominal Si/Al ratios of 100and infinity allowed for the formation of continuous CHA membranes(denoted CHA_100 and _∞, respectively).

The synthesized CHA membranes showed modest CO₂ SFs (5.2 for CHA_100 and7.3 for CHA_∞ at 30° C.) when using dry CO₂/N₂ binary mixtures. However,they showed improved CO₂/N₂ SFs as high as 13.4 for CHA_100 and 9.7 forCHA_∞ at 30° C., when using a wet feed. Such performance improvement wasascribed to the defect blocking by physisorbed water molecules andpronounced for more defective CHA_100. The trend observed for CO₂/N₂separation was similar to that for CO₂/CH₄ separation, but the dry andwet CO₂/CH₄ SFs were comparable to each other. Considering the CO₂ molarflux (relevant to CO₂ recovery) and CO₂/N₂ SF (relevant to CO₂ purity)in a coupled way, CHA_100 showed a marked activity for CO₂ capture inthe temperature range of 30-75° C. (representative of flue gastemperatures). In addition, the degree of CO₂ permeance reduction (75%)due to the presence of water vapor in the feed was much larger for lesshydrophobic CHA_∞ (water contact angle: 25°) than that (39%) observedfor CHA_100 (water contact angle: 80°), supporting that hydrophobicmembrane constituents have desirable roles on membrane performance. Inparticular, the CO₂ molar fluxes through zeolitic parts and non-zeoliticparts could be effectively deciphered by complementing the informationextracted from the image processing of FCOM images with a onedimensional permeation model. This indicates that increasinghydrophobicity was desirable for securing high CO₂ permselectivitiesunder a realistic water vapor-containing feed condition, where defectswith proper size/density could be well blocked by physically adsorbedwater molecules.

INDUSTRIAL APPLICABILITY

According to the present invention, by producing a CHA zeolite membranehaving a high Si/Al ratio, a CHA zeolite membrane having high CO₂/N₂ andCO₂/CH₄ separation performance even under wet conditions can beobtained.

In addition, the CHA zeolite membrane has an effect of exhibiting highseparation performance even under feed conditions containing water,regardless of the temperature and pressure of the flue gas.

Although specific configurations of the present invention have beendescribed in detail, those skilled in the art will appreciate that thisdescription is provided to set forth preferred embodiments forillustrative purposes and should not be construed as limiting the scopeof the present invention. Therefore, the substantial scope of thepresent invention is defined by the accompanying claims and equivalentsthereto.

1. A method of preparing a CHA zeolite membrane comprising: adding asupport including a CHA particle seed layer deposited thereon into asynthetic solution for growing zeolite containing an organicstructure-directing agent and a Si source, without containing an Alsource; and performing hydrothermal synthesis.
 2. The method ofpreparing a CHA zeolite membrane of claim 1, wherein the support isselected from the group consisting of alumina, polypropylene,polyethylene, polytetrafluoroethylene, polysulfone, polyimide, silica,glass gamma-alumina, mullite, zirconia, titania, yttria, ceria, vanadia,silicon, stainless steel and carbon.
 3. The method of preparing a CHAzeolite membrane of claim 1, wherein the organic structure-directingagent comprises one or more selected from the group consisting ofTMAdaOH (N,N,N-trimethyl adamantylammonium hydroxide), TMAdaBr(N,N,N-trimethyl adamantylammonium bromide), TMAdaF (N,N,N-trimethyladamantylammonium fluoride), TMAdaCl (N,N,N-trimethyl adamantylammoniumchloride), TMAdaI (N,N,N-trimethyl adamantylammonium iodide), TEAOH(tetraethylammonium hydroxide), TEABr (tetraethylammonium bromide), TEAF(tetraethylammonium fluoride), TEACl (tetraethylammonium chloride), TEAI(tetraethylammonium iodide), dipropylamine and cyclohexylamine.
 4. Themethod of preparing a CHA zeolite membrane of claim 1, wherein thesynthetic solution for growing zeolite has a molar ratio of TMAdaOH:SiO₂: H₂O: NaOH: Al(OH)₃ of 1 to 100: 100: 1000 to 20000: 1 to 100: 0 to20.
 5. The method of preparing a CHA zeolite membrane of claim 1,wherein the hydrothermal synthesis is performed at a temperature of 100to 200° C. for 12 to 240 hours.
 6. A CHA zeolite membrane prepared bythe method of claim 1, wherein the CHA zeolite membrane has a porosityof a defect of 0.1 to 1% and a z-axis tortuosity (tortuosity in athickness direction of the membrane) of the defect of 1 to
 2. 7. The CHAzeolite membrane of claim 6, wherein the CHA zeolite membrane has aseparation factor of CO₂/N₂ in the flue gas separation processcontaining water, of 10 to 30 and a separation factor of CO₂/CH₄ in theupgrading process of biogas containing water, of 10 to
 80. 8. A methodof separating CO₂ from a mixture containing CO₂ and a molecule selectedfrom the group consisting of CH₄, N₂, O₂, C₂H₄, C₂H₆, C₃H₆ and C₃H₈using the CHA zeolite membrane of claim 6.